1. Field of the Invention
The present invention relates generally to a control system for a power train of an automotive vehicle, and more particularly to an apparatus for preventing occurrence of engine stalling during braking of the vehicle.
2. Description of the Related Art
An automatic transmission in combination with an engine forms a power train of an automotive vehicle and includes a torque converter for absorbing fluctuation in the output torque and multiplying the torque. Since the torque converter performs a power transmission by utilizing a fluid transmission between input and output elements, the transmission efficiency of the torque converter is relatively low. Therefore, it is a recent trend to employ lock-up type torque converters wherein the input and output elements can be locked up, i.e., directly connected with each other, in a driving condition of the vehicle in which the abovementioned functions of the torque converter are unnecessary.
On the other hand, for providing an improved fuel economy, attempts have been made to lower a lock-up vehicle speed and enlarge the lock-up region of the torque converter. The lowered lock-up vehicle speed frequently results in a driving condition in which the vehicle is driven at a low speed with the torque converter locked up. In this instance, when a relatively hard braking is performed, the engine tends to undergo stalling due mainly to a response delay in releasing the torque converter from the lock-up state.
For avoiding occurrence of such engine stalling, there have been various proposals. For example, JP-A-57-161,358 discloses a control system for an automatic transmission, which is capable, in addition to a normal lock-up control of the torque converter, of releasing the converter from the lock-up mode for a period from an instant when the brake pedal is depressed to another instant when an accelerator pedal is subsequently depressed. Further, JP-A-5-193,402 discloses an engine control system wherein an idling speed of the engine is increased during a braking operation.
One may consider it readily possible to combine the two types of controls mentioned above, with an attempt to more positively prevent engine stalling during a braking operation. However, such a combination may deteriorate the drivability of the vehicle particularly when the brake pedal is repeatedly depressed and released for controlling the vehicle speed during a down-hill driving condition. On such occasion, during a period in which the lock-up releasing control of the torque converter is being performed by a transmission controller on the basis of a braking switch signal and an idling switch signal, when the idling speed control of the engine is repeatedly performed by an engine controller upon depression of the brake pedal, the vehicle may be repeatedly subjected to acceleration and deceleration thereby deteriorating the drivability of the vehicle.